Valve size vs. cubic in

Have an interesting technique, tool or just anything related to porting? Post it here . . . please no bashing anyone for the model of engine they are working, we're here to share ideas . . .

Postby hollywood63 » Tue Oct 04, 2005 6:45 am

I'm sure there is a formula here to figure out valve size for a engine but I can't seem to find it. Does anyone have one handy?
Thanks, Art
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Postby gofaster » Tue Oct 04, 2005 1:48 pm

A handy formula for a 2 valve cylinder (one intake and one exhaust) is Intake Valve diameter = 50% to 56% of cylinder bore. That's just a place to start.
You need to know the cfm requirements of the engine in the application you are building for; hot street, circle track, road course, dragstrip, blown, fuel, etc.
Consider the port dimensions and airflow ability you can expect to get from the ports you are working on. A common mistake is to install a valve of much larger diameter than the port can support.
The combustion chamber shape, and the location of the valve in relation to the cylinder wall has a lot of influence on this decision. A large valve may fit but it's effectiveness may be reduced by it's proximity to the cylinder wall or shrouding by the combustion chamber. I've seen situations where the cylinder wall (above the sweep of the top piston ring) was relieved to unshroud the intake valve. Opening up the combustion chamber to unshroud a large valve can turn into a balancing act between making room for the valve and loss of compression ratio.
That's why I said the formula is just a place to start. It helps to have access to other people who have worked on a combination similar to yours. That way you'll have an idea about what does and doesn't work so you don't have to go out there and re-invent the wheel all by yourself.
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Postby cboggs » Sat Oct 08, 2005 2:33 am

I often stay around 52% of the bore, ..

Curtis Boggs
Racing Flow Development.
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